1. Field of the Invention
The present invention relates to a ferroelectric memory and a method for controlling an operation thereof.
2. Description of Related Art
Recently, a non-volatile memory including memory cells composed of a ferroelectric material such as lead zirconate titanate (PZT) having a hysteresis characteristics so that even if a power supply is shut down, a stored content is held. Some examples of this type memory have been reported in Japanese Patent Application Laid-open Publication No. JP-A-63-201998 (which corresponds to U.S. Pat. Nos. 4,873,664), 1988 IEEE International Solid-State Circuits Conference (ISSCC), Feb. 18, 1988, Digest of Technical Papers, pages 130-131, and 1994 IEEE International Solid-State Circuits Conference (ISSCC), Feb. 18, 1994, Digest of Technical Papers, pages 268-269, the disclosure of which is incorporated by reference in its entirety into this application.
Now, based on these reports, a circuit construction and an operation of the conventional non-volatile ferroelectric memory will be described.
Referring to FIG. 1, there is shown a circuit of ferroelectric memory cell, which is disclosed in JP-A-63-201998 and in which one memory cell is constituted of two transistors and two capacitors (called a "2T/2C type" hereinafter). In FIG. 1, Reference Numeral 11 designates a memory cell selection signal line (called simple "selection signal line" hereinafter), and Reference Numeral 13 shows a plate line. Reference Numerals 12 and/12 indicate a pair of complementary signal lines, and Reference Numeral 101 designates a memory cell. Here, in this specification, "/" put just before Reference Numeral such as "12" indicates an upper bar given to the just succeeding Reference Numeral, and means to take a condition complementary to the condition of one given with the same Reference Numeral without "/". Reference Numerals 102 and 103 show switching transistors of the memory cell, and Reference Numerals 104 and 105 indicate ferroelectric capacitors.
In the 2T/2C type memory cell as mentioned above, data is written into the ferroelectric capacitors 104 and 105 in such a manner that the ferroelectric capacitors 104 and 105 always have polarization directions opposite to each other. Electric charges from the ferroelectric capacitors 104 and 105 always have polarization directions opposite to each other, are read out to the pair of data signal lines 12 and/12, so that a voltage difference is generated between the pair of data signal lines, and amplified by a sense amplifier which is composed of a differential amplifier circuit.
Referring to FIG. 2, there is shown a mode of the hysteresis characteristics of the ferroelectric capacitors 104 and 105, which indicates a relation between a spontaneous polarization electric charge Q and a voltage V between opposing electrodes of the ferroelectric capacitor. In addition, a polarization electric charge at the voltage of 0 V, will be called a remnant polarization electric charge Qr. For example, it is considered that when the ferroelectric capacitors 104 and 105 are polarized in conditions A and B, respectively, data "1" is stored, and when the ferroelectric capacitors 104 and 105 are polarized in conditions B and A, respectively, data "0" is stored. At this time, when a voltage of Ve is applied between the opposing electrodes of each ferroelectric capacitor, if the data "1" is stored, an electric charge "Q1" is outputted from the capacitor 104 to the data signal line 12 and an electric charge "Q0 is outputted from the capacitor 105 to the data signal line/12. These electric charges will generate a voltage difference between the pair of data signal lines, as mentioned above. Incidentally, a relation between the remnant polarization electric charge Qr and the output electric charges Q0 and Q1 is ideally expressed as follows: EQU 2.times.Qr=.vertline.Q1-Q0.vertline. (1)
In the above mentioned memory using the ferroelectric capacitor, even if an external voltage applied between the opposing electrodes of each ferroelectric capacitor becomes zero, since the spontaneous polarization internally occurring in the ferroelectric material maintains the data, the data is held after the electric supply is shut down. In other words, so called non-volatile memory operation is realized.
Referring to FIG. 3, there is shown a partial circuit of a memory cell array in a ferroelectric memory using the memory cell of the type shown in FIG. 1. In FIG. 3, Reference Numerals 11A to 11C designate a selection signal line, and Reference Numerals 12A and/12A and 12B and/12B show a data signal line. Reference Numerals 13A to 13C indicate a plate line, and Reference Numeral 14 designates a date signal line precharge control line. Reference Numeral 15 shows a data signal line precharge voltage line, and Reference Numeral 16 indicates a sense amplifier control line. Reference Numerals 101A to 101F each designate a memory cell. Reference Numerals 102A and 103A show a switching transistor of the memory cell, and Reference Numerals 104A and 105A indicate a ferroelectric capacitor of the memory cell. Reference Numerals 106A and 106B designate a data signal line precharge circuit, and Reference Numerals 107A and 107B show a sense amplifier.
Now, a reading operation and a writing operation of the memory cell 101A in the ferroelectric memory will be described with reference to FIG. 3 and FIG. 4 which is a timing chart illustrating an operation of the memory cell shown in FIG. 3. Incidentally, in this specification, unless specially indicated, a logical "high level" correspond to a power supply voltage applied from an device external to the memory, or a voltage generated in an internal voltage generating circuit of the memory, and a logical "low level" is a ground level. Accordingly, these voltages can take various values, for example, 5 V, 3 V, etc., dependent upon the case. Furthermore, for reference, the polarized condition of each of the ferroelectric capacitors 104A and 105A at an end of each of periods (1) to (6) in FIG. 4 is shown below the timing chart in FIG. 4.
In FIG. 4, the periods (1) to (3) show the operation of reading out data from the memory cell. In the period (1), the data signal line precharge control signal 14 is brought to the low level, so as to cancel the precharge condition of the data signal line. Here, the data signal line precharge level is the ground level.
In the next period (2), the selection signal line 11A and the plate line 13A are brought to the high level, so that data is caused to be outputted from the memory cell 101A to the data signal lines 12A and /12A. The data outputted at this time, is determined by the internal polarization condition of the ferroelectric capacitor. The example shown in FIG. 4 shows the reading of the data "1", as will be seen from the explanation made hereinbefore.
In the succeeding period (3), the sense amplifier control line 16 is activated, so that the voltage difference between the pair of data signal line 12A and/12A is sense-amplified.
The periods (4) to (6) illustrate the operation of re-writing the mad-out data to the memory cell. At the time of the period (2), since the data of the read-out memory cell is destroyed, this re-writing is necessary. Incidentally, in the case of writing the memory cell with data supplied from a device external to the memory, it is necessary to set a pair of voltages corresponding to a desired data to be written, onto the pair of data signal lines 12A and/12A during the period (3), before the operation of the succeeding periods (4) to (6) is performed.
During the period (4), the plate line 13A is brought to the low level. In the next period (5), the sense amplifier control signal line 16 is brought to the low level, so as to deactivate the sense amplifier, and furthermore, the precharge control signal line 14 is brought to the high level and the data signal lines are brought to the ground level. With this arrangement, the polarization of the capacitors is returned to the condition of the period (1) before the data reading. Finally, during the period (6), the selection signal line 11A is brought to the low level, so as to render the memory cell transistors (switching transistors) non-conductive. Thus, the access to the memory cell is completed.
When the data "0" is stored in the memory cell 101A, the respective polarization conditions of the capacitors 104A and 105A become opposite to those shown in FIG. 4.
Here, a relation between the above mentioned circuit operation and the characteristics of the ferroelectric capacitor will be discussed. For example, the condition of the period (2) of FIG. 4 in which the selection signal line 11A is brought to the high level so as to turn on the switching transistors 102A and 103A and the plate line 13A is brought to the high level, corresponds to the condition in which the voltage of -Ve is applied to the ferroelectric capacitor (assuming that a direction from the plate line to the data signal line is positive in voltage). At this time, the electric charge Q1 or Q0 is outputted to the data signal line 12A. However, regardless of which of the "1" and "0" is stored, the polarization of the ferroelectric capacitor in this condition is at a point "h" as shown in FIG. 2, and therefore, it is not possible to discriminate "1" or "0". Therefore, it is necessary to re-write the data by applying the voltage +Ve or 0 dependent upon the read-out data "1" or "0", to the ferroelectric capacitor. This operation corresponds to the operation during the periods (4) and (5) in FIG. 4.
As mentioned above, in order to realize the non-volatile memory operation by using the ferroelectric memory cell, it is necessary to apply both positive and negative voltages between the opposing electrodes of the ferroelectric capacitor.
In order to realize a high density of memory, them is a memory cell composed of one transistor and one ferroelectric capacitor (hereinafter, called a "1T/1C type"). This type of ferroelectric memory cell is reported in 1994 IEEE International Solid-State Circuits Conference (ISSCC), Feb. 18, 1994, Digest of Technical Papers, pages 268-269.
Referring to FIG. 5, one example of the 1T/1C type of ferroelectric memory cell is illustrated. In FIG. 5, Reference Numeral 11 designates a memory cell selection signal line, and Reference Numeral 12 indicates a signal line. Reference Numeral 13 shows a plate line, and Reference Numeral 101 designates a memory cell. Reference Numeral 102 shows a switching transistor of the memory cell, and Reference Numeral 104 indicates a ferroelectric capacitor. In the following, it is to be noted that elements corresponding or similar to those shown in the preceding drawings are given the same Reference Numerals, and explanation thereof will be omitted.
In addition, FIG. 6 shows a model of the hysteresis characteristics of the ferroelectric capacitor 104 shown in FIG. 5. In contrast to the 2T/2C type memory cell, in the 1T/1C type memory cell, the two stable conditions "A" and "B" of the ferroelectric capacitor are considered to correspond the data "1" and "0", respectively.
Referring to FIG. 7, there is shown a partial circuit diagram of a memory cell array using the 1T/1C type memory cell. In this case, for example, when the memory cell 101A is selected, a signal voltage from the memory cell appears only on the data signal line 12A. Thus, when the 1T/1C type memory cell is used, it is necessary to generate a reference level used in the sense amplification, on a pairing data line/12A, by circuit of a special means. For this purpose, the circuit shown in FIG. 7 additionally includes reference level generating circuits 108A to 108D and control signal lines 17A and 17B for the circuits 108A to 108D. A specific method for generating the reference level, is disclosed for example in the above referenced 1994 IEEE International Solid-State Circuits Conference (ISSCC), Feb. 18, 1994, Digest of Technical Papers, "Transaction of International Solid-State Circuits Conference (ISSCC)", pages 268-269. The key point of the reference level generation is to generate an intermediate voltage between the data signal line voltage when the signal corresponding to "1" is read out from the memory cell and the data signal line voltage when the signal corresponding to "0" is read out from the memory cell.
Referring to FIG. 8, there is illustrated a timing chart of an operation of the memory cell 101A in the circuit shown in FIG. 7. Furthermore, for reference, the polarized condition of each of the ferroelectric capacitors 104A at an end of each of periods (1) to (6) in FIG. 4 when the data "1" is read, is shown below the timing chart in FIG. 8.
In the case of reading the signal to the data signal line 12A, the reference level generating circuit 108B is controlled to generate the reference level on the pairing data signal line/12A, so that the reference level generated by the reference level generating circuit 108B is read out to the data signal line/12A. The other operation is the same as that of the 2T/2C type memory cell, and further explanation will be omitted for simplification of description.
Furthermore, 1988 IEEE International Solid-State Circuits Conference (ISSCC), Feb. 18, 1988, Digest of Technical Papers, pages 130-131, proposed an example of non-volatile memory in which a ferroelectric capacitor is combined with a flipflop of the type used in a static random access memory (SRAM) (This type memory will be called a "SRAM+ferroelectric memory cell" hereinafter).
Referring to FIG. 9, there is shown a circuit diagram of one memory cell of the "SRAM+ferroelectric memory cell" memory. In FIG. 9, Reference Numeral 18 designates a selection line for an SRAM part, and Reference Numerals 19 and /19 indicate a pair of complementary data signal lines for the SRAM part. Reference Numerals 20 and 21 show a flipflop power supply line. In addition, Reference Numeral 109 designates a flipflop, and Reference Numerals 110 and 111 indicate an N-channel MOS transistor constituting the flipflop. Reference Numerals 112 and 113 show a P-channel MOS transistor constituting the flipflop, and Numerals 114 and 115 show a memory cell selection transistor.
In the memory using this type of memory cell, after the memory is powered on, data is transferred from the ferroelectric capacitor to the flipflop, and when the memory is in a powered-on condition, the memory cell is used as the SRAM cell. Before the memory is powered off, data is transferred from the flipflop to the ferroelectric capacitor, so that after the memory is powered off, the data is held.
Here, causing various elements in FIG. 9 to correspond to various elements shown in FIGS. 1 and 5, the memory cell selection signal line and the data signal lines for the "SRAM+ferroelectric memory cell" memory are the lines 18 and 19 and/19, but the memory cell selection signal line and the data signal lines for the ferroelectric memory are the lines 11 and 12 and/12. In addition, the transfer gate for transferring the data from the ferroelectric capacitor to the data signal line is the transistors 102 and 103. As mentioned hereinbefore, in FIG. 9, elements corresponding or similar to those shown in FIGS. 1 and 5 are given the same Reference Numerals.
Referring to FIG. 10, there is shown a timing chart illustrating an operation of writing data to the ferroelectric capacitors 104 and 105 and an operation of reading data from the ferroelectric capacitors 104 and 105 to the flipflop 109. Correspondence between the polarized condition of the capacitors 104 and 105 and the data "1" and "0" is similar to the case shown in FIG. 2. Furthermore, for reference, the polarized condition of each of the ferroelectric capacitors 104 and 105 at an end of each of periods (1) to (10) in FIG. 10 in the case of writing and reading the data "1", is shown below the timing chart in FIG. 10.
In the case of writing the data from the flipflop 109 to the ferroelectric capacitors 104 and 105, the signal selection line 11 is brought to the high level during the period (1) in which the flipflop 109 holds the data (in the shown example, the data signal line 12 is at the high level and the data signal line/12 is at the low level). In the succeeding periods (2) and (3), the plate line 13 is brought from the low level to the high level, and then, from the high level to the low level. Thereafter, during the period (4), the flipflop power supply line 21 is brought to the low level, so as to power down the flipflop. Lastly, during the period (5), the selection signal line 11 is brought to the low level. Thus, in the capacitors 104 and 105, the polarized condition corresponding to the data stored in the flipflop 109 is set. Thereafter, even if the memory is powered off, the data is held.
In the case of reading data from the capacitors 104 and 105 to the flipflop 109, during the period (6), the selection signal line 11 is brought to the high level, and during the next period (7), the plate line 13 is driven from the low level to the high level, so that a voltage is applied between the opposing electrodes of the capacitors 104 and 105 so as to read an electric charge corresponding to the polarized condition, onto the data signal lines 12 and/12. Thereafter, during the period (8), the flipflop power supply line 21 is brought to the high level to activate the flipflop, so that the signal voltage read out during the period (7) is amplified. During the next period (9), the plate line 13 is returned to the low level, and then, during the period (10), the signal selection line 11 is returned to the low level, to terminate the data reading operation. Thereafter, the memory cell can be used as the conventional SRAM.
In FIG. 10, at the end of the period (1), the polarized condition of the capacitor 15 is indefinite, but at the end of the period (5), the polarized condition is definite. Therefore, it is not a problem that the polarized condition of the capacitor 15 is indefinite at the end of the period (1). In addition, at the end of the period (10), the voltage is applied to the opposing electrodes of the capacitor 104, and therefore, the polarized condition does not correspond to the voltage "0". However, this is not a problem, since the polarized condition is determined at a next time of writing the data. When the data "0" is stored in the flipflop 109, the polarized condition of the capacitors 104 and 105 becomes opposite to the case shown in FIG. 10.
In the shown example, it is possible to use a passive element such as a resistor in place of the P-channel transistors 112 and 113 in FIG. 9.
The above mentioned example adopts a system of reading the data by driving all the plate lines 13 from the low level to the high level so as to apply both positive and negative voltages to the opposing electrodes of the ferroelectric capacitor. However, it is possible to read the data by setting an intermediate voltage on the plate lines so as to apply both positive and negative voltages to the opposing electrodes of the ferroelectric capacitor. Referring to FIG. 11, there is shown a partial circuit diagram of a memory cell array of a memory adopting such a data reading system. In FIG. 11, Reference Numerals 116A and 116B designate a data signal line precharge balance control circuit, and Reference Numeral 22 shows a data signal line balance control signal line. The other construction is the same as that shown in FIG. 7.
Referring to FIG. 12, there is shown a timing chart illustrating an operation of the memory shown in FIG. 11. Here, it is to be noted that the plate line 13 is fixed to an intermediate between the high level voltage and the low level voltage. Now, the reading and writing operations of the memory cell 101A will be described with reference to FIGS. 11 and 12. Furthermore, for reference, the polarized condition of each of the ferroelectric capacitors 104A at an end of each of periods (1) to (7) in FIG. 12 is shown below the timing chart in FIG. 12.
First, during the period (1), the data signal line precharge control signal 14 is brought to the low level, so as to cancel the precharging condition of the data signal line. Here, the precharge level of the data signal line is the ground level, similar to the above mentioned examples. During the next period (2), the selection signal line 11A is brought to the high level, so as to output the data from the memory cell 101A to the data signal line 12A. Here, a point different from the previously described is that the plate line 13 is not driven. Since the precharge level of the data signal line is the ground level and the plate line is an intermediate voltage (called Vm), when the memory cell transistor 102A is rendered conductive during the period (2), a voltage of almost -Vm is applied between the opposing electrodes of the ferroelectric capacitors 104A, assuming that the direction from the plate line to the data signal line is a positive voltage. As a result, a signal voltage corresponding to the polarized condition of the ferroelectric capacitor 104A is read out to the data signal line 12A. At the same time, a reference level is applied to the pairing data signal line/12A from the circuit 10813. In the succeeding period (3), the sense amplifier control signal line 16 is activated to sense-amplify a voltage difference between the pair of data signal lines 12A and/12A.
Incidentally, in the case of writing the memory cell with data supplied from a device external to the memory, it is necessary to set a pair of voltages corresponding to a desired data to be written, onto the pair of data signal lines 12A and/12A during the period (4).
During the period (5), the sense amplifier control signal line 16 is brought to the low level so as to deactivate the sense amplifier. In addition, the data signal line balance control signal line 22 is brought to the high level, so as to bring the data signal line level to the intermediate voltage Vm which is the same as that of the plate line. With this arrangement, the polarization of the memory cell capacitor can be returned to the condition just before the data reading.
During the period (6), the selection signal line 11A is brought to the low level, so as to render the memory cell transistor non-conductive. Thereafter, during the period (7), the pair of data signal lines 12A and /12A are precharged to the ground level. Thus, one cycle of a memory cell access operation is completed.
The signal voltage read out from the ferroelectric capacitor depends upon the magnitude of the voltage applied between the opposing electrodes of the ferroelectric capacitor. Generally, the larger the voltage applied between the opposing electrodes of the ferroelectric capacitor is, the larger the obtained signal voltage becomes. In the operation of the above mentioned ferroelectric memory, the magnitude of the voltage applied between the opposing electrodes of the ferroelectric capacitor depends upon the voltage set on the plate line and the amplitude of the voltage of the data signal line. Accordingly, the set voltage of the plate line and the amplitude of the voltage of the data signal line may be set to any value if they make it possible for the sense amplifier to properly sense-amplify the signal voltage read out from the ferroelectric capacitor. For example, there is a method of making the set voltage of the plate line to one half of the power supply voltage and of making the amplitude of the voltage of the data signal line between the ground voltage and the power supply voltage. Here, the power supply voltage may be supplied from an external of the memory or may be a voltage generated by a voltage generating circuit provided internally in the memory.
Referring to FIG. 13, there is shown a specific circuit of the data signal line precharge balance control circuit 11 6A and 116B. Data signal line precharge transistors 117 and 118 are similar to those shown in FIGS. 3 and 7, and additionally, a data signal line balance transistor 119 is provided between the pair of data signal lines 12 and /12. In a condition that the data signal lines 12 and/12 are at the power supply voltage and the ground voltage, respectively, if the transistor 119 is turned on, since the data signal lines 12 and/12 have the same parasitic capacitance, the data signal lines 12 and/12 become a half of the power supply voltage. This circuit is effective when the set voltage of the plate line is similarly a half of the power supply voltage.
The example shown in FIGS. 11 and 12 has been explained in the case of the 1T/1C type memory cell. However, the ferroelectric memory operating without dynamically driving the plate line is not limited by the type of the memory cell. The 2C/2T type shown in FIG. 3 and the "SRAM+ferroelectric memory cell" type shown in FIG. 9 can similarly operate by drive-controlling the corresponding signal lines similar to the example shown in FIG. 12.
In the examples shown in FIG. 4, 8, 10 and 12, the precharge level of the data signal line is at the ground level. However, the precharge level of the data signal line is not limited to the ground level and may be any value so long as non-zero voltage is applied between the opposing electrodes of the ferroelectric capacitor when the selection signal line 11A is brought to the high level.
However, the above mentioned conventional ferroelectric memory has such a problem that when data is read out from the memory cell, a sufficient voltage is not applied between the opposing electrodes of the ferroelectric capacitor because of the following reasons:
In the ferroelectric memory operating while dynamically driving the plate line as the examples explained with reference to FIGS. 4, 8 and 10 (hereinafter called a "plate drive type"), when the data is read out from the memory cell, the bit line becomes floating. Therefore, when the plate line is driven from the low level to the high level, the voltage of the data signal line varies by action of a coupling through the ferroelectric capacitor of the memory cell. As a result, a voltage not smaller than a coercive voltage Vc converted by multiplying a coercive electric field Ec by a film thickness of the ferroelectric material, is often not applied between the opposing electrodes of the ferroelectric capacitor. Accordingly, the polarization inversion of the ferroelectric material does not occur.
This will be explained in detail with reference to FIG. 14, which illustrates the circuit of the 1T/1C type ferroelectric memory cell. In FIG. 14, a parasitic capacitance of the data line is represented by CD, and the capacitance of the normal dielectric component of the ferroelectric capacitor is represented by CS.
Now, assume that in a condition that the memory cell switching transistor 102 is non-conductive, namely, the memory cell 101 is in a non-selected condition, a voltage VBOOT for turning on the transistor 102 is applied to the selection signal line 11. In addition, the plate line 13 is driven from an initial condition voltage VPL0 to a final condition voltage VPL. Furthermore, it is assumed that an initial voltage and a final voltage of the data signal line 12 are VDL0 and VDL, respectively. Additionally, an initial voltage of a node 23 interconnecting the transistor 102 and the ferroelectric capacitor 104 is VSO and a final voltage of the node 23 becomes VDL since the transistor 102 is turned on.
Under the above mentioned condition, the operation of the memory can be described as follows.
All electric charge Qi of the system shown in FIG. 14 in an initial condition, is expressed as follows: EQU Qi=CS.times.(VS0-VPL0)+CD.times.VDL0 (2)
All electric charge Qf of the system shown in FIG. 14 in a final condition, is expressed as follows: EQU Qf=CS.times.(VDL-VPL)+CD.times.VDL (3)
Since Qi must be equal to Qf, an absolute value .vertline.VPL-VDL.vertline. of the voltage applied between the opposing electrodes of the ferroelectric capacitor is expressed as follows: ##EQU1##
On the other hand, .vertline.VPL-VDL.vertline. must be not smaller than the coercive voltage Vc of the ferroelectric capacitor. The relationship can be expressed: EQU .vertline.VPL-VDL.vertline..gtoreq.Vc. (5)
Here, assuming that the precharge voltage of the data signal line is the ground voltage (namely, VDL0=0) and both of the VS0 and VPL0 are the ground level, the equation (5) can be expressed as follows: ##EQU2##
Here, assume that Vc=1.5 V and VPL=3.3 V, the equation (6) can be expressed as follows: EQU CD.gtoreq.0.833 .cndot. .cndot. .cndot. .times.CS (7)
This equation (7) indicates that since the parasitic capacitance CD of the data signal line has a lower limit, unless the parasitic capacitance is larger than the lower limit, a voltage larger than Vc is not applied between the opposing electrodes of the ferroelectric capacitor. Thus, since the voltage of the data signal line is caused to vary by action of the coupling through the ferroelectric capacitor by dynamically driving the plate line, it is generally said that unless the condition by defined by the equations (4) and (5) is fulfilled, a sufficient read-out voltage cannot be obtained from the memory cell.
On the other hand, in the ferroelectric memory operating without dynamically driving the plate line, as in the examples explained with reference to FIG. 12 (hereinafter called a "plate non-drive type"), a problem similar to that of the plate non-drive type occurs, even if the mechanism causing the problem is different from that in the plate drive type.
In the plate non-drive type of ferroelectric memory, when the memory cell is not accessed, it is necessary to maintain the voltage applied between the opposing electrodes of the ferroelectric capacitor at zero, in order to prevent break of the stored data. In other words, if the plate line is set to the intermediate voltage, a node of one opposing electrode of the ferroelectric capacitor, namely, a node 23 interconnecting the memory cell switching transistor 102 and the ferroelectric capacitor similarly becomes the intermediate voltage. In this condition, if the selection signal line is brought to the high level in order to read out data from the memory cell, first, the electric charge stored in the node 23 interconnecting the memory cell switching transistor 102 and the ferroelectric capacitor similarly becomes the intermediate voltage, is outputted to the data signal line, so that the voltage level of the data signal line changes from the precharge level. As a result, the voltage not smaller than the coercive voltage Vc is not applied between the opposing electrodes of the ferroelectric capacitor, so that the polarization inversion of the ferroelectric material no longer occurs.
This will be explained in detail with reference to FIG. 15, which is similar to FIG. 14, except that the voltage of the plate line 13 is fixed to a constant value VPLC.
Now, consider that an initial condition that the memory cell switching transistor 102 is non-conductive, namely, the memory cell 101 is in a non-selected condition, is changed to a final condition after a voltage VBOOT is applied to the selection signal line 11 so as to turn on the transistor 102.
Except for the condition that the voltage of the plate line 13 is VPLC, the same condition as that explained with reference to FIG. 14 is applied.
All electric charge Qi of the system shown in FIG. 15 in an initial condition, is expressed as follows: EQU Qi=CS.times.(VS0-VPLC)+CD.times.VDL0 (8)
All electric charge Qf of the system shown in FIG. 15 in a final condition, is expressed as follows: EQU Qf=CS.times.(VDL-VPLC)+CD.times.VDL (3)
Since Qi must be equal to Qf, an absolute value .vertline.VPL-VDL.vertline. of the voltage applied between the opposing electrodes of the ferroelectric capacitor in the final condition is expressed as follows: ##EQU3##
On the other hand, .vertline.VPLC-VDL.vertline. must be not smaller than the coercive voltage Vc of the ferroelectric capacitor, similar to the plate drive type, the following relation can be expressed: EQU .vertline.VPLC-VDL.vertline..gtoreq.Vc. (11)
Here, assuming that the precharge voltage of the data signal line is the ground voltage (namely, VDL0=0) and both of the VS0 and VPLC are one half of the power supply voltage Vcc, the equation (11) can be expressed as follows: ##EQU4##
Here, assume that Vc=1.5 V and Vcc=3.3 V, the equation (12) can be expressed as follows: EQU CD.gtoreq.10.times.CS (13)
Similar to the equation (7), this equation (13) indicates that the parasitic capacitance CD of the data signal line has a lower limit. Also in the plate non-drive type ferroelectric memory, it is generally said that unless the condition defined by the equations (10) and (11) is fulfilled, a sufficient read-out voltage cannot be obtained from the memory cell.
The above mentioned discussion is directed to the lower limit of the parasitic capacitance CD of the data signal line. However, in the reading method in which the signal charge read out from the memory cell is outputted onto the data signal line as the signal voltage, if the 2T/2C type memory cell is used, the signal voltage VSIG is expressed as follows by using the electric charges Q0 and Q1 shown in FIG. 2 and the electric charge Qr mentioned hereinbefore: ##EQU5##
Here, the equation (1) mentioned hereinbefore was used.
In addition, in the method of using the 1T/1C type memory cell and also using the reference level generating circuit for generating the reference level, the signal voltage VSIG is expressed as follows by using the electric charges Q0 and Q1 shown in FIG. 2 and the electric charge Qr and the equation (1) mentioned hereinbefore: ##EQU6##
In the above equation, the factor (1/2) means that the reference level is determined to be an intermediate level between the voltage of the data signal line when the data "0" is read out and the voltage of the data signal line when the data "1" is read out. If the reference level generated by the reference level generating circuit is shifted from the intermediate level, the factor in question no longer is 1/2, but a value larger than 0 (zero) but smaller than 1.
Furthermore, the signal voltage VSIG defined by the equations (14) and (15) must exceed the minimum voltage value VSE which can be normally amplified by the sense amplifier. Namely, EQU VSIG.gtoreq.VSE (16)
Namely, the equation (16) means that if the parasitic capacitance CD exceeds a certain value, the signal voltage VSIG becomes too small and therefore smaller than the minimum voltage value VSE which can be normally amplified by the sense amplifier. In other words, the memory cannot operate. Therefore, the parasitic capacitance CD has an upper limit.
Considering the above mentioned matters, the ferroelectric memory generally has a relation between the parasitic capacitance CD and the normal dielectric capacitance CS as shown in FIG. 16. In FIG. 16, the one-dot chain line indicates the lower limit of the parasitic capacitance CD in the plate drive type ferroelectric memory, and the dotted line indicates the lower limit of the parasitic capacitance CD in the plate non-drive type ferroelectric memory. The solid line indicates the upper limit of the parasitic capacitance CD required for obtaining from the memory cell the read-out signal voltage which can be normally data-amplified by the sense amplifier. The hatched region indicates an operation range in which the plate drive type and the plate non-drive type can operate.
In summary, the conventional ferroelectric memories have a problem in which when the data is read out from the memory cell, the voltage of the data signal line varies, although the mechanism of the voltage variation is different from one operation system to another, and in some cases, the coercive voltage Vc required to invert the polarization is not applied between the opposing electrodes of the ferroelectric capacitor, with the result that a normal data reading cannot be performed.